1. Field of the Invention
The present invention relates to an optical element and a method of manufacturing the optical element. In particular, the present invention relates to an optical element having a three-dimensional hollow structure such as a polarizing beam splitter, a phase plate, or a band-pass filter which has on a surface thereof a structure with an in-plane period shorter than a wavelength of visible light, and a method of manufacturing the same.
2. Description of the Related Art
In recent years, optical components having a three-dimensional hollow structure are actively proposed. To obtain such a three-dimensional hollow structure will be essential for improving functions of the optical components in the future. However, a structure of the optical components is on the order of nanometers, a manufacturing method thereof has not been established, and there are many practical problems with regard to strength of the element and the like. In order to obtain such a three-dimensional hollow structure, there is a method using a sacrificial layer to manufacture a hollow structure on the order of micrometers, that is, so-called MEMS (Micro Electro Mechanical Systems) (see U.S. Pat. No. 4,662,746).
Such a MEMS structure provides a digital mirror device. The digital mirror device has a hinge for receiving an operating mirror, a yoke for receiving external forces formed on the hinge, and a mirror for deflecting external light formed on the yoke. This structure is sized to be several microns to several hundred microns, the adhesion between an upper layer and a lower layer is adequate, and no practical problem is caused.
Further, Japanese Patent Application Laid-Open No. 2001-074955 discloses a structure of a photonic crystal waveguide and a method of manufacturing the same. A photonic crystal waveguide is intended to obtain a three-dimensional waveguide by forming structural defects in layers having a line-and-space structure and stacking them in directions orthogonal to one another. According to the manufacturing method disclosed here, a semiconductor material is used to conduct mass transportation of a semiconductor element at a high temperature to form a junction. At such a material junction, metallic bond or covalent bond is possible, and the upper layer and the lower layer can be strongly adhered to each other.
A semiconductor material is transparent in an infrared range but opaque in a visible range, and thus, such a semiconductor material cannot be used for an optical element which functions in the visible range. Therefore, it is necessary to use dielectric materials. However, when dielectric materials are heated to a high temperature, it is sometimes difficult to conduct mass transportation of an element between the dielectric materials to form a junction. In this way, depending on the material, it is sometimes difficult to form a junction by heat. Further, when it is attempted to obtain a stacked bottom-up structure using a sacrificial layer process, in the case of a nanometer structure of the wavelength equal to or less than that of visible light, the contact area between an upper layer and a lower layer becomes extremely small. Therefore, a problem is caused in that the adhesion at the interface between the layers is extremely small and the element is very vulnerable.
The present invention has been made in view of the above-mentioned problems. An object of the present invention is to provide an optical element having a three-dimensional structure which can function in the visible range and can improve the adhesion at a structural interface of the element, and a method of manufacturing the optical element.